In recent years, through optimizing the steel rolling process and improving the performance of the roll device and other technical measures, the hardness, thermal cracking resistance and impact resistance of the roll have been effectively improved and greatly improved, thereby reducing the roll consumption and extending the service life of the roll.
The article focuses on the analysis of irregular block removal in the middle of the rolling groove, cracking of the roll ring, and pitting of the roll groove of hot-rolled narrow strip steel. Improved roll cooling is implemented to adapt to the current situation of the production line. The use of this device and water supply device can effectively improve the cooling effect of the roller, appropriately increase the width of the roll ring, avoid cracks and falling off, and select suitable roller materials.
This system can not only correct the uneven block falling in the middle of the raceway, but also solve the problems of cracks and pitting in the roller ring, and improve the efficient operation efficiency of the roller.
Roller is a very important equipment. Roller is in direct contact with steel billet, and deforms the steel billet by a certain pressure during rolling process, so as to achieve different steel quality standards. Therefore, the fatigue strength and wear resistance of roller are required to be high. Especially in recent years, with the increasing market demand for rolled steel products, the roller consumption in the rolling production process is too large, which in turn increases the production input cost. Under this situation, how to improve the productivity of steel rolling, reduce the frequency of roller replacement, and extend the service life of roller has become a focus of general concern in the metallurgical industry.
Rollers work under high temperature, high pressure, alternating hot and cold, and iron oxide. The rollers of each stand are prone to wear, cracking, peeling, etc. under the action of mechanical stress, friction, thermal stress, and impact load, which greatly shortens the service life of the roller. Roller is an important production link of the rolling mill, and its procurement cost is also relatively high. Roller consumption is an important economic indicator, and it is an important indicator of production cost.
1 Importance of extending roller service life
Through a perfect roll management system, managers can clearly know the condition and trajectory of each roll, make the rolls correctly paired and used, maximize the utilization of the rolls, and formulate appropriate grinding amount, which can not only reduce roll fatigue, make the rolls meet the standards for normal production of the machine, increase the life of the rolls, reduce abnormal roll changes, and improve the production efficiency of the rolling line and increase production benefits.
Whether the roll management system is sound and how well it is managed are key factors in reducing the production cost of the rolling line and extending the service life of the rolls.
A dedicated person is required to manage the rolls, establish a roll management system, and keep records of rolls entering the warehouse for acceptance, paired use, roll cracks, abnormal roll removal, etc. Through a series of such operations, the use track and status of each roll can be managed and controlled, which is crucial to reducing roll consumption and increasing roll service life.
2 Problems with rollers in production
2.1 Irregular block falling in the center of the groove
Too much copper in the center of the groove can easily lead to irregular block drop. Especially when the center of the entire groove is heated, a large amount of copper may flow back and forth on the inner wall surface of the center of the entire groove. Once the copper forms a low-temperature melting point, its thermoplastic properties will be greatly reduced, which will lead to many cracks on the inner surface.
Moreover, as the copper flows back and forth, the cracks will continue to expand and extend, and irregular block drop will occur to a certain extent. Taking hot-rolled narrow strip steel as an example, due to the lack of daily supervision, the rolling amount of each groove is controlled by the finished product adjuster, which leads to inconsistent wear of each groove. When the roll is repaired, the cracks of some grooves are not completely turned off. Due to the existence of micro cracks, when it is put on line again, the micro cracks grow and connect rapidly, eventually leading to the generation of macro cracks, causing the groove to drop.
2.2 Roller ring cracking
Roller rings are subjected to assembly stress, thermal stress and rolling stress during the rolling process. The stress on the roller ring is the algebraic sum of the stress values along the cross section. The roller ring is subjected to large tangential tensile stress on the inner diameter during the rolling process. Using a single material roller ring is not conducive to improving the service life of the roller. At the same time, due to the increase in local stress of the roller ring and the effect of other stresses, the roller ring is also easily damaged and cracked.
2.3 Pitting on the groove
Pitting on the rolling groove is a common surface defect. Its appearance is rough and uneven, also known as “pitting”. Most of them are continuous, but a few are local or sporadic. Pitting is allowed in the rolling groove, but its depth cannot exceed the thickness deviation of the product. The reasons for pitting are:
(1) The finished hole or pre-hole is worn, or there is broken iron oxide.
(2) After being extruded onto the surface of the rolled piece, the broken iron oxide is peeled off.
(3) The rolls are eroded.
(4) The surface of the billet is severely oxidized during heating.
2.4 Roller fracture
During the rolling process, cracks and soft spots are formed on the surface and inside of the roll due to impact, tail swinging, steel clamping, etc. The cracks and soft spots of the roll will seriously affect the normal production of the rolling line and the length of the roll service life. Severe cracks will cause the roll to peel off prematurely, ending the use of the roll prematurely.
3 Cause Analysis
The causes of these problems include: improper human operation, poor cooling effect, technical points of rolled material production, quality problems of the rolls themselves, etc. Improper or inappropriate cooling will produce a large temperature difference on the surface of the roll, thereby accelerating the thermal stress that causes the roll to fall off. At the same time, if the temperature of the roll is too high, it will also affect the strength and wear resistance of the roll, resulting in problems such as cracking, meat loss, burning and even breaking. In production, if the characteristics of the rolled material and production process are not considered, the selected roll material is inappropriate, or the roll is rolled or piled up due to human factors, problems such as cracking, meat loss and roll breakage will also occur.
In actual production operations, since metallurgical steel materials often contain brittle inclusions such as SiO2, Al2O3 or silicates, the presence of these inclusions can easily cause serious impact on the service life of the rolls, and the degree of impact on the life of the rolls varies depending on the number, size and type of inclusions. Generally speaking, the more inclusions there are and the larger their size, the greater the harm, especially the inclusions with sharp edges.
4. Process measures
4.1 Improvement of roll cooling device
In order to improve the utilization efficiency of cooling water and enhance the cooling effect of cooling water, the performance of the roller cooling device can be improved to extend the service life of the roller.
The rolling mill will generate a lot of heat during the rolling process. Therefore, in order to make the replacement cycle and service life of the roller and ensure the temperature after processing, it is necessary to cool down and cool down. The two cooling water tanks of the roller, primary cooling water and secondary cooling water, the primary cooling water is sprayed axially into the rolling area through the slit to lubricate and cool the copper tube; the secondary cooling is to quickly reduce the temperature of the rolling tube, avoid airflow entering the rolling envelope, and prevent the copper tube from oxidizing. The water spray ring of the roller cooling device is used to cool the roller.
During rolling, the main drive speed is above 1300rpm, and the auxiliary drive (roller) is above 700rpm. The roller works at such a high speed, and the flow and pressure of the cooling water need to be strictly controlled. The cooling water must be cooled continuously and fully. If the roller temperature is too high, it must be replaced in time to prevent thermal fatigue cracking.
In actual production, in order to ensure that the temperature of the roller is not too high and maintain the temperature stability, the cooling speed of the roller must be above 3500L/h, and the pressure of the water pump should be below 0.8MPa.
From the original square box-type hole structure to the solid cylindrical nozzle, two rows of elliptical water gaps are added at the cutting wedge part, and the width of the cutting wedge covered by the water gap is 5~8mm; ensuring that the water at the cutting wedge part is 3 times that of other parts, so as to enhance the cooling of the cutting wedge part and improve the rolling groove life of the cutting wedge part.
4.2 Cooling water parameter optimization and water supply system transformation
Choose the appropriate cooling water temperature. If it is too high, it will accelerate the fracture of the roller. If it is too low, it will affect the recrystallization of the rolled tube and the quality of the product. If it is extremely low or high, it will accelerate fatigue. Therefore, in production, the cooling water temperature of the roller should be 40~60℃.
The roller cooling water pipeline has been improved from low-turbid water supply to medium-turbid water supply. The water pump motor uses a variable frequency motor to make the water pressure reach 0.8MPa, and the water pressure can be automatically adjusted.
4.3 Hole configuration optimization
Due to the unsatisfactory cooling water effect, the hot-rolled narrow strip steel φ550 adopts the principle of forced widening on the basis of the original hole type. In view of the high steel temperature of K1, the forced widening pass is set on K3 and K4, and the bottom of the forced widening groove is designed to be a straight line. At the same time, in order to avoid wrinkles on the surface of the rolled piece and cause unqualified surface quality of the finished product, the slope of the forced widening hole is set to 17.7°. After the forced widening is adopted for K3 and K4, the rolling groove size of K6 is changed accordingly to ensure that there is no tail pulling phenomenon.
4.4 Roller material optimization
Taking the commonly used variety of Q215 hot-rolled narrow strip steel as an example, its initial groove center spacing is 19.0mm, and the width of the middle roll ring is 7.8mm, which meets the theoretical design indicators.
However, from the perspective of the overall fracture of the roll ring, the width of the roll ring is still very narrow, which is easy to crack under fatigue conditions, and finally causes the roll ring to crack. For this reason, starting from the perspective of roll length, a roll gap of 22.0mm was designed, and the width of the middle roll ring was adjusted to 10.8mm.
4.5 Automatic thickness control configuration modification
Due to the influence of factors such as inlet thickness, tension, friction coefficient, deformation resistance, etc., the rolling force of the rolling mill will also change accordingly, and the deformation curve related to it will have a certain impact on the thickness outlet.
Automatic thickness control (Automatic Controller Controller, AGC) mainly uses model algorithms to automatically control the strip thickness, rolling equipment conditions, external disturbances, etc., so as to automatically set the juice pressure, speed, and the size of the mill roll gap, and ensure a reasonable thickness deviation.
Usually AGC uses a hydraulic down-pressing system, which has high precision and is relatively fast. The quality control of the rolling mill has a certain relationship with the precise control of the slab thickness, and the number of hydraulic QGC equipment will have a certain impact on the accuracy of the rolling mill. The rough rolling control thickness of the No. 6 steel wire finishing mill is (27~2.7) mm, and the deviation rate is 10%.
4.6 Avoid improper operation
In the production process, roll damage caused by improper use is also very common. Therefore, first, prevent the direct contact between the guide and the roll. Second, strictly implement the cooling water switch of the roll to prevent the head from being dry after replacement. Third, strengthen inspection to prevent problems with the roll. Once a roll occurs, the machine must be stopped immediately and the processing can only be stopped after the winding roll material cools to room temperature. Fourth, strictly control the shape of the rolled material in accordance with the process system to prevent the phenomenon of excessive shape of a single frame. Fifth, the black head at the head of the rolled piece must be cut cleanly, and black steel is prohibited.
4.7 The amount of repair should be in place
In order to ensure that the output of each rolling groove can maintain a relatively stable state, no extra cracks can appear, otherwise the rolling capacity of the rolling mill will be greatly reduced, and if extra cracks appear, it will have a great impact on the quality of the product.
For this reason, during the rolling process, the rolling must be carried out strictly in accordance with the regulations. For example, the turning amount of the finished roll of Q215 hot-rolled narrow strip steel must reach 600mm. If individual rolling grooves cannot be completely eliminated, it can be said that they will no longer be processed or used.
5 Conclusion
By taking the above measures, the service life of the roll can be greatly improved, the number of roll replacements can be reduced, and the labor intensity of workers can be reduced. By correcting the uneven falling of the roll in the middle, the problems of roll ring cracks and pitting can be solved, and the grooves caused by cooling are almost zero, thereby improving the quality of the product. The average effective work efficiency is improved.
